Abstract:

The present study was carried out to test the ability to utilise fish silage to feed
cobia (Rachycentron canadum) in aquaculture in Vietnam. All four experiments of
the present study were carried out over 13 months from June 2006 to June 2007 at
Nha Trang University in Vietnam. The experiments involved making acid-silages
and testing the effects of silage-based diets on digestion, amino acid absorption,
growth and the nutritional quality of the cobia fillets and their shelf life.
The local by-catch fish, lizardfish (Saurida undosquamis) and blue crab (Portunus
pelagicus), were used to prepare 12 uncooked or cooked silages in study I.
Different amounts of formic acid (85%) were mixed with the minced fish and crab
and the silages were stored at ambient temperature (30 ± 2 °C) for 60 days. The
antioxidant effect of adding ethoxyquin was also investigated. At the end of the
experiment, the nutritional quality of all silages was stable. However, cooked silages
showed a tendency for spoilage, particularly cooked crab silages. The antioxidant
effect was only observed in fish-silage groups in which the TBARS values were
significantly higher in the silage without addition of ethoxyquin than in the silage
with it. Generally, the composition of raw materials was reflected in the composition
of the silages. There were significant differences in composition between different
raw material-based silages and between uncooked and cooked silages, particularly
levels of ash, crude protein (CP) and non-protein nitrogen (NPN) at the end of the
trial (for ash: 3.7 – 4.7% in fish vs. 10.7 – 11.5% in crab; for CP: 16.3 – 21.3% in
fish vs. 8.6 – 9.0% in crab; and for NPN: 76.5 – 81.1% in uncooked fish silage vs.
20.0 – 21.3% in cooked fish silage, and 80.1 – 88.9% in uncooked crab silage vs.
54.7 – 57.3% in cooked crab silage). It was concluded that uncooked materials are
more suitable for prolonged storage than cooked materials and it is probably not
necessary to add antioxidants to silages made from materials with a low lipid
content.
All three growth experiments were conducted at the Institute of Aquaculture
Research in Cam Ranh, Nha Trang University, Vietnam. Studies II (cobia size
about 24 g) and III (size about 100 g) were conducted for 6 and 3 weeks, respectively, in indoor tanks supplied with circulated seawater with biological
filtration and constant aeration; experiment IV (size about 500 g) was carried out in
cages in a pond for 3 months.
Lizardfish or blue crab was used to prepare diets for study II, while only lizardfish
was used for study III and IV. Yttrium oxide (Y2O3) was added (100 mg kg-¹ diet) to
the diets for the study of apparent digestibility coefficients (ADC) in study II. Test
diets included either raw or uncooked fish or crab silage as part of the protein
source.
In study II, weight gain (WG) and specific daily growth rate (SGR) were significantly
higher in cobia fed the raw-based diets and fish-silage-based diet (FSD) than in fish
fed crab-related silage-based diets (CSD and MSD) (185 – 286 vs. 34 – 90% and
2.49 – 3.21 vs. 0.68 – 1.53%/day). Feed conversion ratios (FCR) were significantly
higher in the groups fed CSD and MSD diets than the groups fed the other diets
(2.06 – 6.49 vs. 0.97 – 1.16), resulting in significantly lower protein productive
values in the groups fed CSD and MSD than in the other groups (0.06 – 0.16 vs.
0.31 – 0.37). The FCR results were confirmed by significantly lower ADC values in
fish fed CSD and MSD than in fish fed the other diets. The difference in the growth
of fish fed raw or silage diets was probably related to the distribution of amino acids
in the plasma and liver.
Study III showed significantly higher concentrations of most free essential amino
acids at 6 and 12 h, but significantly lower levels at 48 h post-feeding in fish fed
silage-based diets than in fish fed raw-based diet. At 48 h post-feeding, the total
level of plasma free amino acids (FAA) was significantly higher in fish fed rawbased
diet than in fish fed silage-based diets (4999 vs. 3390 – 4339 nmol AA ml-¹
plasma). Similarly, at 48 h post-feeding FAA concentrations in the liver were
significantly higher in fish fed 0%- or 13%-silage-based diets than in fish fed 26%-
or 39%-silage-based diets. Faster amino acid absorption in the diets containing
higher levels of fish silage led to a significantly higher concentration of plasma FAA
earlier, which could result in an imbalance of amino acid concentrations for protein
synthesis later on and eventually affect growth rate. In study IV, no significant differences in nutritional composition were observed
between the fillets of cobia fed raw- or silage-based diets for 3 months. Cobia fillets
contained of a balance of amino acids (EAA/NEAA=1). The three groups of fatty
acids; saturated fatty acid (SFA), monounsaturated acid (MUFA) and
polyunsaturated fatty acid (PUFA) in cobia fillet were divided into quite similar
proportions from 30 to 33% of TFA. PUFA accounted for 30% of total fatty acids
with high levels of n-3 PUFA (69% of total PUFA). n-3 PUFA consisted mainly of
docosahexaenoic acid (DHA) (46 – 49% of total PUFA) and eicosapentaenoic acid
(EPA) (11 – 12% of total PUFA). The quality index method (QIM) was used to
estimate the shelf life of whole gutted cobia stored in ice. QI scores and quantitative
descriptive analysis (QDA) from both groups were correlated throughout storage (r²
= 0.83 – 0.86). However, the total scores for QIM and QDA were low compared to
the maximum scores given in the schemes (14 vs. 24 and 5 vs. 10, respectively)
after 15 days storage in ice. In addition, the development of lipid oxidation and the
growth of bacteria in cobia fillets were also below the acceptable limits after 15 days
storage in ice. Thus, the final overall shelf life of the cobia was not determined in
the present study, but it is at least 15 days.
In order to make efficient use of local by-catch fish and by-products from the
processing industry for aquaculture in general and for cobia in particular, more
studies on the preparation of silage and proportions used are needed, since
nutritional demand varies between life stages and from species to species.
Moreover, further studies on shelf life and the processing quality of cobia are also
needed.